Screen printing, also known as serigraphics, is the process of transferring an image to a substrate by the use of a printing screen through which ink is squeezed. The ink is then deposited in all places on the substrate except where the screen has been processed by a photographically applied image depicting the places where ink is not to be forced through the screen mesh. The imaged screen mesh is normally made of silk, plastic or metal and is held in place by a screen frame made of wood, plastic or metal. The ink contains pigment or dye in an appropriate vehicle.
Screen cleaning requires the removal of all ink residue from the screen and frame when the printing is completed. Methods currently utilized to clean printing screens involve spraying the screen with pressurized solvent from a nozzle or gun structure. Many spraying techniques, however, result in overspray which directs solvent where it is not desired thereby wasting solvent and increasing the costs and inefficiency of the procedure. Additionally, spray cleaning methods usually require the use of paid workers thus raising the costs of cleaning and also exposing the workers to the solvent and solvent fumes.
Several attempts have been made to develop a cleaning procedure and apparatus which eliminates the various drawbacks of the above-mentioned procedures. For example, U.S. Pat. No. 3,580,261 discloses a printing screen cleaner which utilizes two vertically oriented lines of nozzles which oppose each other inside a cleaning enclosure. A screen is passed on a conveyer belt through the opposing nozzle lines whereupon it is sprayed with pressurized solvent. While eliminating several of the drawbacks of the above-discussed existing methods, the cleaner has other drawbacks which prevent the efficient and effective cleaning of a printing screen. Specifically, it utilizes only two opposing lines of spray nozzles within the enclosure and, therefore, each area of the screen is sprayed only once as the screen passes through the opposing nozzle lines. The screen is generally not adequately cleaned when the cleaning cycle has been completed and, therefore, the screen must be taken and passed through the cleaner additional times for additional cleaning cycles until it is sufficiently clean. As may be appreciated, such a task is time consuming and costly, and requires continuous worker supervision of the cleaning machine. For example, the work involves handling the screen by insertion into the machine, removal and manual brush agitation, then reinsertion, to insure adequate cleaning. Therefore, the cleaner disclosed in the '261 patent is not very efficient or cost effective.
The screen washing apparatus of U.S. Pat. No. 3,656,493 utilizes an enclosure and a single spray nozzle which is directed over the screen by a control mechanism to spray a predetermined pattern. Specifically, the nozzle sweeps horizontally in one direction across the screen to an end position and is then ratcheted down a few rows of the screen whereupon it sweeps in the opposite direction to an opposite end position and is ratcheted down to begin the process again. The nozzle continuously sweeps and ratchets until it reaches a bottom position which is pre-set by an operator. The screen is swept only once by the nozzle and if any portions remain dirty, the operator must remove the nozzle and manually spray the screen. Furthermore, the single nozzle only sprays a small area of the screen at any given time. Again, such a screen washing apparatus is neither efficient nor cost effective because the cleaning process has to be monitored to ensure that the single sweep was sufficient to remove the printing ink and the screen must be manually sprayed if the single sweep was not sufficient. Furthermore, the gun is controlled with manual pre-sets which must be adapted to spray a particular screen size, and insertion of a different size screen into the washing apparatus requires additional programming of the spray pattern of the device, thus further reducing the efficiency and cost effectiveness of the apparatus. Additionally, the operator is exposed to solvent fumes whenever it is necessary to manually direct the gun to completely clean the screen.
The screen cleaning apparatus of U.S. Pat. No. 4,420,004 utilizes a single row of adjacent nozzles which moves horizontally with respect to a screen to spray the screen and remove the printing ink as well as the stencil or print pattern. The single row of spray nozzles requires a continuous back and forth sweeping motion of the nozzle row which must be repeated until the screen is adequately cleaned. Since there is only a single line of nozzles, only a small area of the screen is sprayed at any one time leaving the remaining area unsprayed until the nozzle line again passes over that area. This increases the time that is required to adequately clean the screen because when one small area of the screen is being sprayed, the remaining area of the screen is dormant. As a result, the cleaning cycle of the apparatus in the '004 patent is inefficient and, therefore, not cost effective.
In view of the above background, there is a need for a screen cleaning apparatus which rapidly and efficiently cleans a printing screen and frame with little or no supervision by operators and other personnel, thereby making the apparatus very cost effective. Such a cleaning apparatus should also adequately and efficiently clean a screen in a single cleaning cycle without requiring removal of the screen and re-insertion into the cleaning apparatus for continual repetition of the cleaning cycle. It would be highly desirable to provide a cleaning apparatus which catches and reuses the sprayed solvent while maintaining a closed spray environment to prevent the escape of solvent and solvent fumes into the surrounding area or atmosphere.